This invention relates generally to blood vessel imaging, and more particularly the invention relates to forward viewing intravascular ultrasound imaging.
Coronary artery disease is the number one cause of mortality in the United States and peripheral vascular disease remains a major cause of morbidity. Percutaneous interventions have rapidly developed to address the blockages in the blood vessels which result in angina, heart attacks and limb ischemia. In 1990 greater than 300,000 coronary angioplasties were performed in the United States. The methods for addressing these blockages include balloon angioplasty as well as many newer technologies such as excimer lasers, directional coronary atherectomy and high speed rotational burrs. The traditional and still primary method for guiding these interventions in angiography. Angiography is limited to defining the size and course of the lumen of the blood vessel and therefore gives little information about the actual structure and geometry of the blockage and the blood vessel wall. Because of this limited image guidance and primitive intervention devices, the incidence of acute complications remains significant, with a 3 to 5% rate of myocardial infarction and 1 to 2% death rate. More importantly, the lack of adequate visualization results in inadequate removal of the blockage and may contribute to the high rate of recurrence.
Newer methods of visualization of the blood vessel have become available in the past few years. Angioscopy allows visualization of the optical characteristics of the surface of the blockage but gives no information about the underlying shape and structure of the blockage. Furthermore, angioscopy requires large amounts of flushing to keep the field of view clear. Thus, angioscopy remains a poor method for guiding intervention.
Intravascular ultrasound has many of the properties of an ideal system for evaluating blockages and other lesions in blood vessels. The creation of images based on echo delay times results in visualization of the internal structure of the blockages and other lesions in blood vessels. The creation of images based on echo delay times results in visualization of the internal structure of the blockage and of the arterial wall. Furthermore, since blood is relatively echolucent, no flushing is required to maintain an image, therefore continuous imaging during intervention is feasible.
The current generation of intravascular ultrasound devices are all essentially side looking devices. As such, the device must be passed through the blockage in order for it to visualize the blockage. Since the smallest of the current generation of devices is 2.9 Fr (1 mm in diameter), the ultrasound catheter usually cannot be advanced through a significant blockage without disturbing it. In the case of complete occlusions, the ultrasound catheter cannot be used at all.
A forward looking ultrasound device, that is a device which is not restricted to side looking, would permit the evaluation of blockages without disturbing them and potentially serve as a useful tool for guiding recanalization of complete occlusions. The need for such a device has been discussed for many years. Some degree of forward imaging has been proposed in the past by angling the mirror used to redirect the ultrasound beam so that a conical section is obtained, rather than the radial slice that results from a typical side looking transducer. The conical sections obtained by this approach are not well suited for assessing the degree of atherosclerosis or for assessing the size of the lumen.
An implementation of a true forward viewing sector scanner was recently described which uses a complex mechanical linkage to achieve the forward scanning. The complexity of this approach, however, has resulted in a bulky device which measures 4 mm (14 Fr) in diameter. A device of this dimension, although possibly suitable for use in the peripheral vasculature, could not be used in the coronary circulation.
In order to achieve the goal of a catheter suitable for evaluating coronary arteries as well as peripheral vessels, the device dimensions should be such that it will fit comfortably in a vessel 3 mm in diameter. Therefore, the catheter diameter should be less than 2 mm and ideally under 1.5 mm. Furthermore, to provide useful images, the device will ideally provide 1 cm of penetration to permit complete visualization of most blockages and provide a 50 degree scan sector so that the scan will subtend a typical 3-5 mm diameter vessel.
The present invention is directed to a mechanical sector scanner for achieving these goals.